1. Field of the Invention
The present invention relates to propylene polymerization. In particular, the present invention concerns a process for preparing homopolymers and copolymers of propylene in a reactor system comprising a combination of at least one slurry reactor and at least one gas phase reactor. The present invention also relates to an apparatus for producing homo- and copolymers of propylene.
2. Description of Related Art
A large number of processes for preparing propylene homo- and copolymers are known in the art. Thus, for example, when MgCl2 +TiCl4, a conventional, supported high-yield catalyst is used for polymerization, numerous different kinds of slurry and gas phase processes can be employed. The bulk process is a slurry process, wherein the reaction takes place in pure monomer or in a reaction medium containing more than 60 wt-% of the monomer. The main advantage of a bulk process is formed by the high catalyst activity due to the fact that polymerization takes place in liquid monomer. This means that to achieve commercially acceptable catalyst productivity (expressed as kg polymer/gram of catalyst), a short residence time in the reactor is sufficient. Short residence time in the reactor means that the reactor can be of small size compared to e.g. fluid-bed gas phase reactor. The small reactor size leads to relatively low polymer inventory in the reactor, which speeds up transitions between different products.
The gas phase processes have lower activity because polymerization takes place in gaseous monomer. This leads to longer residence times, which increases the reactor size and thus polymer inventory required leading to slower grade transitions. On the other hand, the investment cost of gas phase processes is lower (less complicated), especially due to lower unused monomer recycle leading to lower recovery equipment investment costs. Another advantage of gas phase processes is the possibility to produce high comonomer content products. Still another advantage is the better inherent safety of the gas phase processes due to lower monomer inventory and lower pressure compared to bulk processes. In order to draw benefit from and avoid draw-backs of the different features of slurry bulk and gas phase processes, combined bulk and gas phase processes have been suggested in the prior art.
For polymerization of vinyl chloride there has been proposed a slurry/gas phase reactor cascade where the first reactor is a loop reactor, and the polymer content of the loop reactor is concentrated with settling legs and led to a second reactor, which is a fluidized bed reactor. Reference is made to U.S. Pat. No. 3,622,533. The polymerization is continued in the fluidized bed. The outlet is made discontinuously using a settling leg in the loop in order to minimize the reaction medium transported to the gas phase reactor.
Polymerization of propylene is a loop reactor, which can be operated in supercritical conditions, is disclosed in U.S. Pat. No. 4,740,550. The product of the loop reactor is conducted to a gas phase reactor, wherein the reaction is continued. Before entering the gas phase the fines fraction of the polymerization product of the loop reactor is removed and fully or partly circulated back to the loop reactor. Together with the fines, a part of the unreacted monomers from the gas phase reactor is recycled directly to the 1st stage loop reactor.
The main object of U.S. Pat. No. 4,470,550 is to provide a process for preparing a block copolymer of high quality by feeding homopolymer with narrow residence time distribution to the block copolymerization stage. The process disclosed comprises the following stages: first stage homopolymerization in a bulk, loop reactor, fines removal cyclone between the first and second stage, second stage homopolymerization in a gas phase reactor and finally impact copolymerization in an additional gas phase reactor.
One problem with the process in U.S. Pat. No. 4,740,550 is that if all the fines removed from the first stage loop reactor outlet are circulated back to the loop reactor, there is a risk that eventually the loop reactor is filled with inactive dead catalyst or slightly polymerized dead fines. On the other hand if a portion of this fines stream is combined with the product from the last reactor, this might cause inhomogenity problems in the final product. Still further if a portion of this fines stream is collected separately and blended with a separate homopolymer product as also suggested in U.S. Pat. No. 4,470,550, this leads to complicated and economically unacceptable operation. As will be discussed in the detailed description of the present invention, we have found that impact copolymer of high quality can be produced with two-stage homopolymerization followed by an impact copolymerization step without any fines removal and circulation either after the first or second stage homopolymerization.
In the present invention one of the main objects is to minimize the amount of circulation by a specific sequence of reactors and by selecting the relative amounts produced in each reactor with that object in mind. This is an idea, which is clearly not the object of U.S. Pat. No. 4,740,550. This point is further clarified in the detailed description of the invention and in the examples.
For polymerization of olefins a process is known in which the first reaction is made in liquid, and the second reaction in the absence of the liquid (cf. GB Patent No. 1 532 231).
A two-step process has also been suggested for polymerization of ethylene, cf. U.S. Pat. No. 4,368,291.
A slurry prepolymerization connected to the gas phase process is proposed in WO 88/02376.
A gas phase process for polyolefins, where a special catalyst with spherical form is employed, has been proposed in EP-A 0 560 312 and EP-A 0 517 183. The catalyst is prepolymerized in a loop reactor using more than 5 parts polymer and 1 part catalyst up to 10 wt-% of total production.
JP Patent Applications (Laid Open) Nos. 58/065,710, 01/263,107 and 03/084,014 describe the manufacture of propylene-ethylene block copolymers in an apparatus comprising a combination of a slurry reactor and a gas phase reactor. The polymer slurry from the slurry reactor is fed into a classifying system installed between propylene polymerization vessels, and a slurry containing coarse particles is supplied to a flash for gas separation and polymer is then fed to an ethylene-propylene copolymerization vessel, while the slurry containing fines is returned to the slurry vessel.
Some of the disadvantages associated with bulk and gas phase processes, respectively, are avoided by the suggested prior art combination processes. However, none of them meets the requirements for flexibility and low production costs dictated by the commercial production configuration. The recycling of large amounts of unreacted monomers from the the second stage reactor back to the first stage slurry (bulk) reactor increases investment and production costs and prevents independent control of reaction medium composition in the two reactors.
It is an object of the present invention to eliminate the problems related to the prior art of single and multiple-reactor processes and to provide a novel process for preparing homopolymers and copolymers of propylene (and other alfa-olefin(s)).
It is another object of the invention to provide a highly versatile process which can be used for preparing a wide range of different homopolymer and copolymer products of propylene.
It is a third object of the invention to provide a novel apparatus for producing homo- and copolymers of propylene.
These and other objects, together with the advantages thereof over known processes, which shall become apparent from specification which follows, are accomplished by the invention as hereinafter described and claimed.
The process according to the present invention is based on a combination of at least one slurry reactor and at least one gas phase reactor connected in series, in that order, to form a cascade. Propylene homo- and copolymers are prepared in the presence of a catalyst at elevated temperature and pressure. According to the invention, the polymerization product of at least one slurry reactor, containing unreacted monomers, is conducted to a first gas phase reactor with minimum or no recycling of monomer back to the slurry reactor.
The homo- or copolymers prepared in the combination of the slurry and first gas phase reactor are homophasic, i.e. miscible, and any rubbery component is added later.
According to another aspect of the invention, at least one slurry reactor and at least one gas phase reactor connected in series are employed as a reactor system, the at least one slurry reactor being a bulk loop reactor operated at high or super critical temperature, and the content of the slurry reactor, including the polymer product and reaction medium containing unreacted monomers, being directly fed into the gas phase reactor fluidized bed.
According to still a further aspect of the invention, the reactor product of at least one slurry reactor is subjected to product separation by reducing the pressure thereof to evaporate volatile components. The solid substances of the product separation operation are conducted to the gas phase reactor. The evaporated reaction medium including the unreacted monomers are separated from the other volatile components and also fed to the gas phase reactor, whereas hydrogen and inert hydrocarbons (e.g. lower alkanes), if any, are removed. The separated stream can be used in further reactors, e.g. as a hydrogen rich stream instead of hydrogen feed, or the hydrogen can be recovered for other purposes.
The apparatus comprises a reactor cascade formed by at least one slurry reactor connected in series with at least one gas phase reactor together with a conduit interconnecting said one slurry reactor with said one gas phase reactor for conducting essentially all of the unreacted monomers from the slurry reactor to the gas phase reactor.
More specifically, the process according to the present invention is mainly characterized by what is stated in the body of claim 1.
The invention achieves a number of considerable advantages. With the present arrangement it has been found that the monomer fed into the first reactor can, to a large extent or fully, be consumed in the gas phase reactor(s) after the slurry reactor. This is possible due to gas phase operation with small amount af gas leaving with the polymer product. The loop reactor dynamics in the cascade provides fast transitions and high productivity. Fast start-ups are also possible because the gas phase bed material is available directly from the loop reactor. With the loop and gas phase reactor cascade it is possible to produce a large variety of different broad molecular weight distribution or bimodal products. The at least one gas phase reactor provides high flexibility in the reaction rate ratio between the first and second part of the product because of adjustable bed level and reaction rate. The gas phase reactor has no solubility limitations which makes it possible to produce polymers of high and very high comonomer content.
Furthermore, one of the preferred embodiments depicted in FIG. 3 below, which comprises separation of light components before the recovered monomer is fed into the gas phase, makes it possible independently to control polymerization conditions in slurry and gas phase, respectively, and thus provides for maximum flexibility of polymer alloy preparation.
In summary, by means of the present invention it is possible to provide:
A. A process for preparing standard and novel homopolymers and copolymers of propylene;
B. A process with minimum or no recycling of monomer(s) back to the first stage reactor leading to cost effective production;
C. A process, which produces standard homopolymer, propylene-ethylene random copolymer and propylene-ethylene impact copolymer grades at similar or lower cost than best state of the art technology;
D. A process, which produces novel propylene homopolymer, propylene-alfa-olefin random copolymer, propylene-ethylene-alfa-olefin terpolymer and propylene-ethylene-(alfa-olefin) impact copolymer grades at a cost similar or not significantly higher than the production of corresponding standard grades by the best state of the art technology;
E. A process, where high productivity, fast dynamics and compact reactor size of the 1st stage is combined by direct feed with the high once-through conversion, product and residence time flexibility and feed monomer evaporation power of the second stage;
F. A process as stated in E combined with a 3rd stage impact copolymerization reactor and further to provide such a product transfer system between stages 2 and 3 that possible excess hydrogen coming from stage 2 can be removed before stage 3;
G. A process as stated in E with a possibility to prepare novel polymer grades with broad molar mass distribution and/or high comonomer(s) content;
H. A process as stated in F with a possibility to prepare novel polymer grades with broad molar mass distribution and/or high comonomer(s) content;
I. A process, where high productivity, fast dynamics and compact reactor size of the 1st stage is combined via separation unit with the high once-through conversion, product and residence time flexibility and feed monomer evaporation power of the second stage;
J. A process as stated in I combined with a 3rd stage impact copolymerization reactor and further to provide such a product transfer system between stages 2 and 3 that possible excess hydrogen coming from stage 2 can be removed before stage 3;
K. A process as stated in I, where the composition of the reactors can be controlled largely independently allowing a possibility to prepare highly novel polymer grades;
L. A process as stated in J, where the composition of the reactors can be controlled largely independently allowing a possibility to prepare highly novel polymer grades; and
M. A process as stated in any of the objectives above, where the 1st stage reactor is operated at high or supercritical temperature to increase productivity, to improve heat removal and to provide a compact reactor size.